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Query: UMLS:C0155339 (Brown)
 12,436 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Primary points of control in steroidogenesis are the transport of cholesterol from intracellular stores to the inner mitochondrial membrane, and the subsequent conversion of cholesterol to pregnenolone by the cholesterol side-chain cleavage enzyme (P450scc). Testosterone production has been shown to decline in Brown Norway rat Leydig cells as the rats age. To better understand the mechanism by which aging Leydig cells lose steroidogenic function, we examined the effect of aging on steroidogenic acute regulatory protein (StAR), an important Leydig cell cholesterol transfer protein, and on P450scc. Leydig cells isolated from middle-aged (14 months) and old (24 months) rats produced significantly less testosterone than cells from young (4 months) rats. StAR mRNA (1.7 kilobase [kb]) was significantly reduced in Leydig cells from middle-aged and old rats, by 26% and 52%, respectively. Significant reductions also were seen in the steady-state levels of mRNA for P450scc, of 29% and 50%, respectively. Western blots revealed significant reductions in StAR protein, by 47% and 74%, respectively, and in P450scc protein, by 38% and 54%, respectively. In response to LH stimulation in vitro, testosterone production by Leydig cells in young, middle-aged, and old rats increased by 30-, 40-, and 33-fold, respectively, although the amounts of testosterone produced by the young cells significantly exceeded that produced by the middle-aged and old cells. StAR protein also increased in response to LH by 1.4- , 3-, and 11-fold, respectively, whereas P450scc protein remained unchanged. These results are consistent with the conclusion that compromise of StAR-mediated cholesterol transport may play a key role in age-related reductions in Leydig cell steroidogenesis. However, because P450scc is reduced in old Leydig cells, the reaction catalyzed by this enzyme would be rate-limiting under circumstances in which saturating amounts of cholesterol entered the mitochondria.
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PMID:Leydig cell aging: steroidogenic acute regulatory protein (StAR) and cholesterol side-chain cleavage enzyme. 1119 Oct 81

The cellular mechanisms responsible for age-related decline in the ability of Leydig cells to produce testosterone are not yet fully understood. The decline in testosterone production could result from a reduction in the Leydig cell enzymatic activities mediating testosterone synthesis, the amount of substrate available for these enzymes, or both. In the present study, we examined the effect of age on a critical early step in the steroidogenic pathway, the transport of cholesterol into mitochondria. Leydig cells were isolated from the testes of young and old Brown Norway rats and incubated with human chorionic gonadotropin (hCG) and the side-chain cleavage cytochrome P450scc inhibitor aminoglutethimide (AMG). Mitochondria were isolated from these cells in the presence of AMG. Upon removal of AMG, the mitochondria from old cells produced 80% less steroid than those from young cells, only a fraction of which could be accounted for by a decrease in P450scc activity. These results suggest that the accumulation of hormonally recruited cholesterol into mitochondria is defective in old Leydig cells. With this in mind, we turned our attention to peripheral benzodiazepine receptor (PBR), a mitochondrial cholesterol-binding protein known to be involved in mediating cholesterol transport. PBR messenger RNA (mRNA) and protein expression were decreased in old cells. Moreover, both the dissociation constant (Kd) and the number of binding sites (Bmax) of the PBR were decreased in the old cells by 50% and 30%, respectively. Taken together, these results suggest that alterations in cholesterol transport and in PBR may play critical roles in age-related decreases in testosterone production in Brown Norway rat Leydig cells.
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PMID:Cholesterol transport, peripheral benzodiazepine receptor, and steroidogenesis in aging Leydig cells. 1200 32

The wealth of knowledge about the function and regulation of adult Leydig cells, the cells within the mammalian testis that produce testosterone, make these cells ideal for studying principles and mechanisms of aging. A hallmark of mammalian aging is decreased serum testosterone concentration. In the Brown Norway rat, this has been shown to be associated with the reduced ability of aged Leydig cells to produce testosterone in response to LH. Herein, we demonstrate that culturing the aged cells with dibutyryl cAMP, a membrane-permeable cAMP agonist that bypasses the LH receptor-adenlyly cyclase cascade, restores testosterone production to levels comparable to those of young cells and also restores steroidogenic acute regulatory protein and P450scc, the proteins involved in the rate-limiting steps of steroidogenesis. These results strongly suggest that signal transduction deficits are responsible for reduced steroidogenesis by aged Leydig cells and that bypassing signal transduction reverses the steroidogenic decline by the aged cells.
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PMID:Dibutyryl cyclic adenosine monophosphate restores the ability of aged Leydig cells to produce testosterone at the high levels characteristic of young cells. 1523 95

Previous studies have shown that the capacity of Leydig cells from aged (21-24-month-old) Brown Norway rats to produce testosterone is reduced from young (4-month-old) levels, and that this is correlated with reductions in steroidogenic acute regulatory protein (StAR), peripheral benzodiazapine receptor (PBR), and the levels and activities of the steroidogenic enzymes. The age(s) at which particular changes in the steroidogenic pathway occur, and the relationship of particular changes to reduced testosterone production, are not known. We examined 3 critical components of the steroidogenic pathway, cyclic adenosine monophosphate (cAMP) production, StAR, and P450 side-chain cleavage enzyme (P450scc) in relationship to age-related decreases in testosterone production. Leydig cells isolated from Brown Norway rats of increasing ages (4, 9, 15, and 20 months) were evaluated. The ability of Leydig cells to produce testosterone was reduced at 9 months, although not significantly. Significant reductions in testosterone production were first seen in cells isolated from rats of 15 months of age, and further reductions occurred thereafter. Reduced testosterone was correlated with reductions in StAR, P450scc mRNA, and protein. Significant decline in luteinizing hormone-stimulated intracellular cAMP levels was seen by 9 months, before significant reductions in testosterone, StAR, and P450scc. Further declines in cAMP levels were seen at 15 and 20 months. These studies suggest that age-related reductions in intracellular cAMP may lead to the reduced testosterone production that characterizes aged Leydig cells. This suggestion is supported by recent studies from our lab demonstrating that long-term (3 days) culture of old Leydig cells with dbcAMP restored testosterone production to levels approximating those of young cells.
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PMID:Temporal relationships among testosterone production, steroidogenic acute regulatory protein (StAR), and P450 side-chain cleavage enzyme (P450scc) during Leydig cell aging. 1561 63